Operation and performance of the PEP-II prototype longitudinal damping system at the ALS

Abstract

A modular programmable longitudinal feedback system has been developed as a component of the PEP-11 R+D program. This system is based on a family of VME and VXI packaged signal processing functions which implement a general purpose digital feedback controller for accelerators with bunch spacings of 2 ns. A complete PEP-11 prototype system has been configured and installed for use at the LBL Advanced Light Source. The system configuration used for tests at the ALS is described and results are presented showing the action of the feedback system. Open and closed loop results showing the detection and calculation of feedback signals from bunch motion are presented and the system is shown to damp coupled-bunch instabilities in the ALS. Use of the system for accelerator diagnostics is illustrated via measurement of grow-damp transients which quantify growth rates without feedback, damping rates with feedback, and identify unstable modes. PEP-11 Longitudinal PrototypeThe PEP-11 longitudinal prototype is a modular bunch-by-bunch processing system designed to implement longitudinal feedback signal processing [l]. The system is composed of several processing modules in VME and VXI formats. The analog-to-digital and digital-to-analog functions are implemented in a pair of VXI processing modules which are capable of sampling bunch motion and generating correction signals at a 500 MHz rate, which allows the system to operate with bunch spacings of 2 ns. The 500 MHz A/D functions are incorporated in the Downsampler module, which is a programmable sequencer capable of controlling the sampling of up to 2048 bunches with downsampling factors up to 32. The 500 MHz DIA is contained in the hold buffer VXI module, whose buffer memory contains the most recent kick value for each bunch in the system. A VXI packaged QPSK modulator (which generates a kicker carrier signal which spans 1 to 1.25 GHz) and a Timing module (which generates several ECL clock signals from the ring RF master oscillator} complete the digital signal processing VXI modules [2].The signal processing algorithms are implemented as programs coded on AT+T DSP 1610 microprocessors. These single-chip 16 bit processors are organized into groups of four, *Work supported by Department of Energy, contract DE-ACO3-76SFOO515with four processors on each V M E packaged DSP module. A system configuration for a particular accelerator requires 3 to 20 of the DSP modules, depending on the machine synchrotron frequency, revolution frequency and number of bunches. Each sampling interval the Downsampler module sends bunch data to a particular DSP board (based on the downsampler program) and the DSP board sends computed results to the Hold buffer module. Figure 1 is a photograph of the system configured for testing at the ALS, which uses 10 DSP modules total.The system is configured and downloaded via a set of control programs which specify the operating configuration of the system and generate coefficients for the DSP algorithms. An EPICS based user interface allows a single co...

Review of Feedback Systems

“…Current analog to digital converters are able to convert every 2 ns an analog signal into an 8 bit word. Detector systems with such ADC's have been tested successfully for the PEP B factory feedback [20] so there are now detectors available which can deliver every 2 ns a new digitised value for the oscillation amplitude. If the bunch spacing is larger than 2 ns 12 or even 14 bit ADC's can be used which may be important for feedback systems in proton machines to reduce digitising noise.…”

“…This procedure is called down sampling and is used in the design of the longitudinal feedback of the PEP B factory [17]. This method has been successfully tested in a prototype in the ALS [20]. After digital processing the data is converted into a analog signal using fast digital to analog converters.…”

Enhanced functionality and performance of the longitudinal damping system at the ALS using a new VXI based processing system

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AbstractThe ALS longitudinal feedback electronics, designed and developed at the Stanford Linear Accelerator center (SLAC), has been operating very reliably at the ALS for over four years [1]. Recently a new VXI based implementation of the entire RF and timing electronics has been built, tested, and will soon replace an earlier prototype installation.

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BESSY II feedback systems

“…In addition to these measures, a longitudinal feedback system (LFB) will be installed in summer 1999. The LFB electronics developed for the ALS (Berkeley), PEP-II (Stanford) and DAऄNE (Frascati) [7,8] will be used in combination with a kicker cavity designed for DAऄNE [9] and modified for a bunch frequency of 500 MHz [10]. For details on the kicker cavity see [11].…”